1. Field of the Invention
This invention relates to methods of and apparatus for coding digital data, and to apparatus for decoding digital data. The invention is more particularly, but not exclusively, concerned with the coding of channel status information data and user information data in the serial transmission of audio data.
2. Description of the Prior Art
Referring to FIG. 1 of the accompanying drawings, in the AES/EBU (Audio Engineering Society/European Broadcasting Union) format for the serial transmission of audio data, the audio signal to be transmitted is sampled at 48 kHz and is coded into 32-bit audio data words, each of which in fact consists of a 4-bit frame synchronizing signal, four auxiliary bits, twenty audio data bits, and a further group of four bits consisting of a validity bit, a user bit, a channel status information bit and a parity bit. The bits are recorded according to the bi-phase mark rule, except for the 4-bit synchronizing signals, which violate the rule and are so recognizable on decoding. The 4-bit synchronizing signals enable the 32-bit audio data words to be identified and correctly framed on reception. The channel status information bits from 192 successive 32-bit audio data words form a 192-bit channel status information word. Likewise, the user bits from an aligned 192 successive 32-bit audio data words form a 192-bit user word. The channel status information words and the user words are correctly framed on reception by making the 4-bit synchronizing signal in the 32-bit audio data words containing the first bit of each channel status information word and user word different from the 191 succeeding 4-bit synchronizing signals.
Briefly, the channel status information may relate to audio attributes, program attributes and transmission attributes. The user words are usable at the option of a user to convey such information as the user may wish.
It will be appreciated that in the above-described format, the channel status information words are rigidly fixed within the overall block structure. In some ways this is advantageous, because so long as the incoming signal for decoding is correctly received, the channel status information words can be reliably extracted and decoded according to fixed rules determining the position and significance of bits within the channel information status words. However, the incoming signal for decoding is not always correctly received.
FIG. 2 of the accompanying drawings shows a very simplified example of this. At the top is shown an analog audio signal which is continuous throughout, and which has been subjected to digital coding according to the format described above. Below is shown the basic structure of the resulting digital signal, each rectangle representing a block of 192 of the 32-bit audio data words including a single channel status information word. Any one of a number of faults, such as a transmission fault or a drop-out can cause a jump in the blocking, that is, a discontinuity in the block structure, whereupon the reliability of the channel status information is destroyed until the block structure has been recovered.
Likewise, the channel status information will be lost or corrupted in a number of quite normal circumstances, such as where there is a change in the frequency at which the audio signal is sampled, where there is a cross-fade between two audio signals, where there is mixing of two audio signals, or generally where there is editing of the audio signal, or even where there is a change in the speed at which the audio data is reproduced.
The problems which are referred to above and which may arise in connection with the channel status information words may similarly arise with the user words. Moreover, the proposal contained herein for alleviating these problems in respect of the channel status information words can likewise be used for the user words, although for convenience it is the channel status information words which will be mainly referred to in what follows.